1. Field of the Invention
The invention relates to a ghost detector for detecting a reflected wave (so-called "ghost" which is generated upon signal reception in television broadcasting) from buildings or other obstacles that is superimposed on a direct wave. More particularly, the invention relates to a ghost detector suitable for use in signal reception by mobile receivers. The invention also relates to a ghost suppressor for suppressing the unwanted effects of ghost.
2. Discussion of the Related Art
A conventional circuit for removing reflected wave components from the signal received by a television receiver is well known as a "ghost canceler" and used for the purpose of signal reception by fixed receivers. In such a ghost canceler, a reference signal (GC signal) for vertical blanking is extracted from a video signal and any distortion present in the waveform of the received signal is corrected by a suitable apparatus such as a transversal filter so that the reflected component which was undesirably added to the reference signal is reduced to zero.
According to this conventional method of suppressing reflected waves, the reference signal for vertical blanking exists only for an extremely short time within a field period (1/60 second in the NTSC system) and the magnitude of the reflected wave component and the time of reflection are compensated, i.e., controlled to be reduced, for every 1/60 second, requiring a certain time, typically 3 to 4 seconds, for the reflected wave component to level off toward zero. When electric waves for television broadcasting are to be received by a fixed receiver, several seconds may well be taken to suppress the reflected wave and, hence, the above-described conventional method will cause no practical problems. However, this conventional method which takes time in ghost detection is unable to measure the actual generation of ghost during signal reception by a mobile receiver, making it impossible to realize a high-speed ghost canceler.
A further problem with the conventional system is that if a SAW (Surface Acoustic Wave) convolver is directly used as a correlating device, the time required for the convolver to achieve correlation is very short, only a fraction of the length of scanning lines for television and, as a result, it has been impossible to detect ghosts of short delay time. If the time required for the convolver to achieve correlation is prolonged, the propagation loss which is caused during the correlating operation will increase to an impractical level.